sort_characters_by_frequency.py

"""
Given a string s, sort it in decreasing order based on the frequency of the characters. The frequency of a character is the number of times it appears in the string.

Return the sorted string. If there are multiple answers, return any of them.

 

Example 1:

Input: s = "tree"
Output: "eert"
Explanation: 'e' appears twice while 'r' and 't' both appear once.
So 'e' must appear before both 'r' and 't'. Therefore "eetr" is also a valid answer.
Example 2:

Input: s = "cccaaa"
Output: "aaaccc"
Explanation: Both 'c' and 'a' appear three times, so both "cccaaa" and "aaaccc" are valid answers.
Note that "cacaca" is incorrect, as the same characters must be together.
Example 3:

Input: s = "Aabb"
Output: "bbAa"
Explanation: "bbaA" is also a valid answer, but "Aabb" is incorrect.
Note that 'A' and 'a' are treated as two different characters.
 

Constraints:

1 <= s.length <= 5 * 105
s consists of uppercase and lowercase English letters and digits.

"""

class Solution:
    def frequencySort(self, s: str) -> str:
        count = Counter(s) # Char -> Count
        buckets = defaultdict(list) # freq -> [char]
        
        for char, cnt in count.items():
            buckets[cnt].append(char)
        
        res = []
        for i in range(len(s), 0, -1):
            for c in buckets[i]:
                res.append(c*i)
        return "".join(res)